NEW YORK -- There's a reason behind Gary Sheffield's perpetual assignment as an Interleague Play designated hitter, beyond his experience with the job. Bothered by pain behind his right knee for the past two weeks, Sheffield has been avoiding the field in an effort to heal.

It's worked, if only to an extent. Sheffield has felt markedly better in recent days, and his offense, not coincidentally, has improved. But that won't stop him from undergoing an MRI after this weekend's Subway Series to determine the extent of whatever damage might exist.

"He's struggled the last week or so with that," Mets manager Jerry Manuel said.

Due to recent injuries and illnesses to outfielders Carlos Beltran, Ryan Church and Angel Pagan, Manuel hasn't been able to rest the 40-year-old Sheffield nearly as often as he planned to do after Sheffield signed with the team. Instead, Sheffield has played close to every game with the Mets, jumping out to a hot start before falling into a 3-for-31 funk earlier this month.

Sheffield's long home run on Friday night at Yankee Stadium snapped him out of that slump -- he's hit a single, a double and another long home run in seven at-bats since that first homer. And after Saturday's 6-2 win, Sheffield credited his rejuvenated legs, citing an earlier inability to attack breaking pitches.

"I think not running around in the outfield, my legs aren't getting fatigued," Sheffield said. "I feel like I have my legs back under me."

Still, Sheffield's right knee has bothered him, prompting the Mets' desire for an MRI. Sheffield is tentatively scheduled to remain the DH for next week's three-game series in Baltimore, another American League city. But the results of his MRI should determine whether Sheffield can or cannot play defense at Citi Field next weekend, and whether the Mets will keep rookie Fernando Martinez on the roster or not.

By then, the Mets hope that Sheffield will have healed, making Martinez expendable.

"I think the DH, this has come at a good time for us," Manuel said.